The present invention relates to vibration welding or more particularly to vibration welding of thermoplastic joints.
The ongoing demand to use thermoplastics to replace metals in automotive vehicle applications such as air induction systems has increased in recent years. It is estimated that by the year 2010, 21.4 million air intake manifold components will be produced using welding techniques. This invention concerns an improved method of vibration welding thermoplastic joints, and the vibration welded articles produced by the method. Vibration welding of thermoplastics such as nylon 6 and nylon 66 is well known in the art. See V. K. Stokes, "Vibration Welding of Thermoplastics, Part I: Phenomenology of the Welding Process", Polymer Engineering and Science, 28, 718 (1988); V. K. Stokes, "Vibration Welding of Thermoplastics, Part II: Analysis of the Welding Process", Polymer Engineering and Science, 28, 728 (1988); V. K. Stokes, "Vibration Welding of Thermoplastics, Part III: Strength of Polycarbonate Butt Welds", Polymer Engineering and Science, 28, 989 (1988); V. K. Stokes, "Vibration Welding of Thermoplastics, Part IV: Strengths of Poly(Butylene Terephthalate), Polyetherimide, and Modified Polyphenylene Oxide Butt Welds", Polymer Engineering and Science, 28, 998 (1988) and C. B. Bucknall, et al, "Hot Plate Welding of Plastics: Factors Affecting Weld Strength", Polymer Engineering and Science, 20, 432 (1980).
Vibration welding may be conducted by vibrating two parts under pressure along their common interface to generate frictional heat, and thereby melting and fusing their surfaces together. Vibration welding is a quick and inexpensive way to join irregularly shaped parts of various sizes. In the past, vibration welding has been used in low load-bearing applications. In automobile underhood applications such as air intake manifolds, air filter housings, and resonators, the expanded use of engineered plastics would be desirable to achieve savings and weight and cost. However, heretofore it has not been possible to achieve adequate weld strengths for such uses. Welding results are extremely sensitive to parameter uniformity and slight variations in them can result in significant changes in weld quality. Vibration welding parameters of pressure, frequency, amplitude, oscillation (welding) time, hold time and weld thickness all affect tensile strength of welds. It is an object of the present invention to provide a method for vibration welding fiber reinforced thermoplastic surfaces to provide welds having greater tensile strengths than have been heretofore achievable.
U.S. Pat. No. 4,844,320 teaches that weld strength are not affected by weld amplitude or weld time above a certain level. In conventional vibration welding, welds are formed at a vibration amplitude of 0.03 to 0.70 inch. In contrast we have found that weld amplitude and weld time are extraordinarily important criteria for increasing weld strength. The vibration welding of this invention uses a vibration amplitude of at least about 0.075 inch. It has been determined that conventional vibration welds of reinforced thermoplastic surfaces achieve a maximum tensile strength of approximately 80% of a weld formed by the unreinforced surfaces of corresponding thermoplastic materials. For glass fiber reinforced thermoplastics, this lowered tensile strength is attributed to a change in the glass fiber orientation at the welded joint. According to this invention, vibration welds of reinforced thermoplastic surfaces achieve a maximum tensile strength of as high as about 120% of a weld formed by the unreinforced surfaces of corresponding thermoplastic materials. This is because fibers from at least one of the surfaces are caused to penetrate both into the weld and into the other surface. This provides added, unexpected tensile strength to the weld.